THE final proof of whether or not a lubricating oil is suitable for use, comes only as the result of engine tests conducted under conditions simulating those the oil must meet in service. To be of value, these tests must be carefully controlled and the results recorded in a form that is reproducible.
In the case of piston-skirt deposits, it has been found extremely difficult to secure accurate quantitative results, reproducible by different observers; and to obtain a permanent record of the form, location, and intensity of the deposits.
The authors' work on piston-skirt deposits has led them to develop two devices, which they have described here in detail.
The first of these makes use of a reflection densitometer to obtain readings of the reflection density of a large number of points on the piston to be studied. These readings are converted to reciprocal reflectivity values, the average of these values being called the piston rating. A year's experience with this method has shown that the results are reproducible by different observers and are most helpful in evaluating the piston deposits.
The other device described is a piston photographing machine, which attempts to overcome the difficulties inherent in the photography of cylindrical objects. The apparatus is based on the principle that the piston, when rolling in a circle, at some time presents each point of its circumference equidistant from the center of the circle. The instrument gives a photograph of the entire piston surface developed upon a plane with practically no distortion. Even photographs in color can be obtained, thus allowing direct comparison not only of amount and type of deposit but also of the color. Simplicity of operation and uniformity of the conditions for piston lighting and exposure are the main advantages of this machine.
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